CVD processes are used to modify the surface properties of substrates by depositing one or more layers of a metal or refractory or metallic compound through chemical reactions occurring in a gaseous medium surrounding the substrate. Physical vapor deposition processes such as EBPVD also modify the surface properties of substrates, but typically no chemical reaction occurs at the substrate or in the gas phase; the mechanism is one of condensation of the coating constituents from the vapor phase onto the substrate to form the coating rather than chemical reaction of gas phase precursor materials.
In chemical vapor deposition (CVD) processing, a substrate or part is heated to an elevated temperature (usually greater than 800.degree. C. and frequently as high as 2000.degree. C.) and exposed to a mixture of reactant gases. The reactant gases are selected to include compounds of the specific atomic species desired in the finished coating. (Metal halides and hydrides are commonly-used reactants.) Chemical reactions occur at the surface of the substrate to yield the desired coating composition. Pyrolysis, reduction, oxidation, hydrolysis and coreduction are among the types of reactions that have been utilized commercially: